High-energy heavy-ion (A–A) collisions offer a unique possibility to study nuclear matter under extreme temperature and density, in particular the properties of the deconfined quark–gluon plasma (QGP) [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, which was predicted by quantum chromodynamics (QCD) [8]}, [9]}, [10]}, [11]}, [12]}. The interpretation of the heavy-ion results depends crucially on the comparison with results from small collision systems such as proton–proton (pp) or proton–nucleus (p–A).
Measurements in pp collisions establish a reference for larger systems and are used to test perturbative QCD models. The p–A collisions, which are intermediate between pp and A–A collisions in terms of system size and number of produced particles [13]}, [14]}, [15]}, are traditionally used to separate initial and final-state effects [16]}, [17]}.
However, at the LHC the pseudorapidity density (d\(N_{\rm ch}\) /d\(\eta \) ) of final-state charged particles in pp and p–A collisions can reach values comparable to those achieved in semi-peripheral Au–Au [18]} and Pb–Pb collisions [19]} at the top energies of RHIC and the LHC, respectively.
Therefore, there exists a possibility of final-state effects due to the formation of dense matter even in p–A collisions.
